1. Field of the Invention
Embodiments of the invention relate to a display device, and more particularly to, a liquid crystal display (“LCD”) device and fabricating method thereof. Although embodiments of the invention are suitable for a wide scope of applications, it is particularly suitable for improved transmittance in an LCD device.
2. Discussion of the Related Art
Liquid crystal displays (“LCDs”) control electric fields applied to liquid crystal cells to modulate light incident to the liquid crystal cells, thereby displaying an image. LCDs are classified into either vertical electric field-type LCDs or horizontal electric field-type LCDs, depending upon the direction of an electric field that drives the liquid crystal material.
In a vertical electric field-type LCDs, when a voltage is applied to pixel electrodes and common electrodes opposing each other on upper and lower substrates, an electric field is applied across the liquid crystal material between the electrodes. Vertical electric field-type LCDs have the disadvantage of a narrow viewing angle.
In a horizontal electric field-type LCDs, when a voltage is applied to pixel electrodes and common electrodes arranged on the same substrate, an electric field is applied across the liquid crystal material between the electrodes. Horizontal electric field-type LCDs have the advantage of a wide viewing angle, as compared to vertical electric field-type LCDs.
Horizontal electric field-type LCDs include a thin transistor substrate joined to a color filter substrate such that the substrates face each other, spacers to maintain a cell gap between the two substrates and liquid crystal material within the cell gap. The thin film transistor substrate includes signal lines and thin film transistors to generate a horizontal electric field in each cell, and an alignment film is positioned over the signal lines and the thin film transistors for aligning the liquid crystal material. The color filter substrate includes color filters to render colors, a black matrix to prevent light leakage and an alignment film applied is positioned over the color filters and the black matrix for aligning the liquid crystal material.
FIG. 1 is a view illustrating a thin film transistor substrate of a horizontal electric field-type LCD according to the related art. As shown in FIG. 1, the related art LCD thin film transistor substrate includes: a gate lines 2 and data lines 4 crossing each other, to define pixel regions; thin film transistors 6 at each crossing of an associated one of the gate lines 2 and an associated one of the data lines 4; first common lines 16a and second common lines 16b each extending in parallel to the gate lines 2 in the pixel regions; common electrodes 18 each connected to the first common lines 16a while extending over the pixel region with finger portions 18b; and pixel electrodes 14, extending over the pixel region, that are individually connected to a drain electrode of an associated one of the thin film transistors 6 and alternately arranged with finger portions 18b of the common electrodes 18.
The first and second common lines 16a and 16b are formed at the same time as the gate lines 2 using the same non-transparent metal as the gate lines 2. The first and second common lines 16a and 16b are connected to the common electrodes 18 and supply a common voltage to the common electrodes 18.
The liquid crystal display shown in FIG. 1 further includes connecting lines 16c to connect the first non-transparent common lines 16a with the second non-transparent common lines 16b. The connecting lines 16c extending in parallel to the data lines 4 and are made of a non-transparent metal like the first and second common lines 16a and 16b to prevent light leakage of the pixel regions during driving of the liquid crystal display.
In response to a scan pulse of the gate line 2, the thin film transistor 6 applies a data signal from the data line 4 to the pixel electrode 14 in the pixel region. For this operation, the thin film transistor 6 includes a gate electrode 8 connected to the gate line 2, a source electrode 10 connected to the data line 4 and the drain electrode 12 connected to the pixel electrode 14. The thin film transistor 6 further includes an active layer (not shown) forming a channel between the source electrode 10 and the drain electrode 12 above the gate electrode 8, and ohmic contact layers (not shown) to allow ohmic connection to the active layer by the source electrode 10 and the drain electrode 12.
The common electrode 18 is connected to the first common line 16a through a contact hole 17, and includes a base portion 18a extending in parallel to the gate line 2 and a plurality of finger portions 18b extending from the base portion 18a into the pixel region. The common electrode 18 is made of a transparent metal.
The pixel electrode 14 includes a first pixel electrode 14a connected to the drain electrode 12 of the thin film transistor 6 though the contact hole 13 and extending in parallel to the gate line 2, and a plurality of second pixel electrodes 14b extending from the first pixel electrode 14a to the pixel region and being arranged alternately with the finger portions 18b of the common electrode 18. The pixel electrode 14 is made of the same transparent metal as the common electrode 18. The first pixel electrode 14a overlaps the second non-transparent common lines 16b with an insulating layer (not shown) to form a storage capacitor.
A horizontal electric field can be applied between the second pixel electrode 14b that receives a data signal through the thin film transistor 6 and the finger portion 18b of the common electrode 18 that receives a common voltage through the first common line 16a. This horizontal electric field leads to rotation of liquid crystal molecules that were in initially aligned in a horizontal direction in the pixel region due to dielectric anisotropy. Further, the transmittance of light transmitted through the pixel region is varied depending on the degree of rotation of the liquid crystal molecules such that a gray scale can be implemented. However, in related art LCDs, transmittance deterioration occurs at an end portion of the second pixel electrode 14b and at an end portion of the finger portion 18b of the common electrode 18, as shown in regions A and B of FIG. 1.
FIG. 2 is a view illustrating the phenomenon of transmittance deterioration occurring in region A when an electric field is applied. As shown in FIG. 2, the liquid crystal molecules 20 in the region A are driven not only by an electric field applied between the finger portion 18b of the common electrode 18 and the second pixel electrode 14b, but also by an electric field applied between the base portion 18a of the common electrode 18 and the second pixel electrode 14b. Meanwhile, polarizing plates with transmission axes crossing at right angels, to control light transmittance, are respectively mounted in upper and lower parts of the liquid crystal display. During driving of the liquid crystal molecules 20, in the case of the regions A and B where the transmission axis of the polarizing plates do not correspond to the alignment of the liquid crystal molecules 20, light is not transmitted, as compared to the remaining regions, and therefore contrast and brightness degrades.